A building block of integrated circuits is the transistor. A transistor can be implemented in both planar and non-planar topologies. A planar transistor generally includes a channel region within a semiconductor substrate. A gate stack including a gate electrode over a gate dielectric is provisioned directly over the channel. Gate spacers are provisioned to either side of the gate stack. Source and drain regions are provisioned to each side of the channel, and may extend under the corresponding gate spacer, or even the gate dielectric, in some cases. Non-planar transistor topologies are typically based on a fin structure, and generally include so-called FinFETs and nanowire transistors.
A FinFET is a transistor built around a thin strip of semiconductor material (generally referred to as the fin) that extends from an underlying substrate. The transistor includes the standard field-effect transistor (FET) nodes, including a gate, a gate dielectric, a source region, and a drain region. The conductive channel of the transistor effectively resides on sides of the fin beneath the gate dielectric. Specifically, current runs along/within both sidewalls of the fin (i.e., on sides perpendicular to the underlying substrate surface) as well as along the top of the fin (i.e., on a side parallel to the underlying substrate surface). Because the conductive channel of such configurations essentially resides along the three different outer, planar regions of the fin, such a FinFET design is sometimes referred to as a tri-gate transistor. Other types of FinFET configurations are also available, such as so-called double-gate FinFETs, in which the conductive channel principally resides only along the two sidewalls of the fin (and not along the top of the fin). A nanowire transistor, sometimes referred to as a gate-all-around transistor, is effectively a fin that has a relatively low aspect ratio because some underlying portion of the fin is removed so that the gate stack material can surround the channel region on all sides.